Many industrial processes require vacuums in order to work. For example, in the manufacturing of polymers, foodstuffs, moldings and the like a vacuum is often required to initiate or sustain a chemical reaction, drive off excess moisture, to de-gas materials or to assist in the forming and shaping of manufactured products. In many cases, vacuums are used in conjunction with wet chemical processes, which leads to partial evaporation of the chemicals involved, which in turn means that gases drawn from the processes can contain contaminants.
An industrial pumping system will often comprise a number of pumps connected in series or in parallel to provide increasingly higher vacuum pressures from one pumping stage to the next. A dry vacuum pump, that is one that is designed to pump gases without using a sealing liquid in the vacuum pump swept volume, generally comprises one or more rotors arranged to cooperate with a stator: movement of the rotors causing the gas to be drawn into the pump and pushed through it towards an exhaust port of the pump. In order that the pumped gas does not backflow, and to improve the pumping efficiency of the pump, the gaps between the rotors and stators are minimized—usually to within a few hundred microns.
A problem therefore arises when the gas being pumped contains contaminants, which can build up on the surfaces of the rotors and stators thereby closing the gap between them. When a certain thickness of contaminants has built-up on the surface of the rotors and stators there is a chance that the pump will jam preventing further operation or restarting without remedial maintenance.
In most situations, vacuum pumps run hot, that is the act of compressing the gas being pumped causes heating of the gas, which heat is transferred to the rotors and stators of the pumps. In such a situation the majority of the contaminants within the gas being pumped will not condense on the rotors and stators and will simply pass through the pump. Moreover, because the pump components are hot, in use, contaminants that are viscous liquids will generally remain in a low-viscosity state thereby enabling the pump to continue running. However, when the pump is switched off, the contaminants may have a tendency to thicken or harden upon cooling, thereby preventing the pump from being restarted, even though, when hot, it was able to run.
In the alternative situation, that is where the pump components exceed certain temperatures, there is a chance that contaminants within the pumped gas will react to form hard compounds that may build up on the rotor and stator surfaces. Hard compounds on the rotor and stator surfaces can cause excessive or premature wear of the pump components, thereby degrading the efficiency of the pump or even leading to catastrophic failure.